The present invention relates generally to a steering system with a variable steering ratio, and more particularly to a steering control apparatus and method for diagnosing cable attachment in a steering system with a variable steering ratio.
Conventionally, there is known a steering system for vehicle, which includes a steering shaft equipped with a spiral cable, such as a spiral cable for airbag. Such a spiral cable has one end fastened to a vehicle body, and the other end fastened to a steering shaft, being wound around the steering shaft with a margin. After attachment of this spiral cable, a steering wheel connected to the steering shaft is turned lock to lock (specifically, turned rightward until the steering wheel is locked or restricted, and then turned leftward until the steering wheel is locked), in order to check whether the spiral cable is correctly attached with a margin for right-and-left rotation of the steering wheel. Since a typical steering system includes no member to directly restrict movement of the steering shaft, the state of lock is defined in case of a rack-and-pinion steering mechanism by a condition that a contact portion of a rack shaft is mechanically restricted by a stopper to the vehicle body, or is in contact with the stopper. Even when the check of cable attachment is inadequate, for example, when there is no margin for rightward rotation of the steering wheel, the spiral cable for airbag is easily ruptured by a driver's force of operating the steering wheel. Thus, in the above-mentioned steering system, no undesirable steering lock occurs during the vehicle driving.
A Published Japanese Patent Application No. 2003-324836 (referred to as “JP2003-324836”) shows a spiral cable device for a variable steering ratio steering system for an automotive vehicle. The spiral cable device shown in JP2003-324836 includes a cylindrical housing, an inner cylinder provided so as to relatively rotatable with respect to the housing, and a flexible flat cable having one end connected to the inner cylinder, and having the other end connected to the hosing. The variable steering ratio steering system shown in JP2003-324836 includes an electric motor as an actuator for regulating its steering ratio, so as to variably control the steer angle of a steered road wheel with respect to the steering shaft angle. The stator of the electric motor is fixed to the steering shaft, and connected electrically to electrical equipment mounted in the vehicle body via the flexible flat cable as in the case of the spiral cable for airbag. Even when the stator rotates with rotation of the steering shaft, the spiral cable allows transmission of electric power and signals.